Vertical integration of a micro-electro-mechanical systems (MEMS) device assumes that the MEMS structure is anchored to both top and the bottom substrate, i.e. handle, or cover, wafer and an applications specific integrated circuit (ASIC) wafer. Building mechanical structures on silicon wafer based on the deposition and etching of different structural layers is called surface micromachining. In surface micromachining usually a sacrificial layer is deposited on a substrate followed by a deposition of mechanical layer where the moving parts of the accelerometer are going to be defined. The moving parts are later released by selectively removing the sacrificial layer. This method has many shortcomings for building low cost and high performance accelerometers. For example, there are contradicting requirements over the area (cost) of the accelerometer and the noise performance. The Brownian noise level of the accelerometer is proportional to the size of the proof mass. In surface micromachining, the proof mass height is determined by the deposited film thickness which is usually limited to less than 10 microns. Therefore, building heavy proof masses requires relatively large area which in return increases the cost.
Surface micromachining also necessitates complex fabrication steps. Depositing thick films which are required for low accelerometer noise is a very sophisticated process. Moreover, non-uniformity of the deposited films and large variation of the material properties have negative impact on the process yield and cost. Controlling stress level in the film is another issue which needs to be dealt with. Otherwise undesired curling of the released structures may occur. In addition, moveable parts released by using sacrificial wet etching may suffer from the stiction problem if their mechanical properties are not selected properly. Stiction can be avoided by fabricating structures with high spring constants. But this adversely affects the sensitivity of the accelerometer where the sensitivity is inversely proportional to the resonant frequency. Therefore, stiction problem limits the accelerometer sensitivity.
In addition to the above described technical difficulties, surface micromachining tools are not readily available to small companies. Most of the required equipment can only be supported by a complicated infrastructure that only large companies can afford. This sets a very high barrier for small start-up companies that want to enter the accelerometer market. Surface micromachining is not a feasible solution for companies which do not have access to the expensive fabrication equipment.
Bulk micromachining, on the other hand, overcomes most of the technical difficulties of surface micromachining as well as providing a viable solution for fabless semiconductor MEMS companies. In contrast to surface micromachining, bulk micromachining defines structures by selectively etching the substrate. Since the height of the structures is defined in the substrate, it is possible to build accelerometers with increased height and reduced foot print without the complexities associated with building structures using deposited layers. Increased mass in a small footprint provides fabricating the accelerometer with better noise performance at a reduced cost. In addition, bulk micromachining techniques are readily available through MEMS foundaries. Bulk micromachined devices can easily be built on off the shelf SOI (silicon on insulator) substrates.
Another important process step for fabricating a low cost MEMS device is the integration of mechanical parts with the electronics. To address this need a “Nasiri-Fabrication” platform is utilized which is described for example in (U.S. Pat. No. 7,104,129, entitled “Vertically integrated MEMS structure with electronics in a hermetically sealed cavity”) and assigned to the assignee of this application. This fabrication process makes use of bulk micromachining and readily allows for the water level integration of the MEMS substrate and the electronics (ASIC) substrate. In addition to integration, this method encapsulates the mechanical parts in a low pressure hermetically sealed chamber that protects the MEMS device against adverse effect of environment such as humidity.
The Nasiri fabrication platform essentially requires a two-sided anchor. This type of anchor has both advantages and disadvantages over traditional one-sided anchors. To describe these features refer now to the following description in conjunction with the accompanying Figures.
The basic steps of Nasiri-fabrication are shown in FIGS. 1A-1H. A handle wafer 10 is etched to form cavities as shown in FIG. 1A and FIG. 1B. Oxide is then grown on the handle wafer 10. A handle wafer 10 and a device wafer 100 are then fusion bonded together as shown in FIG. 1C. The handle wafer 10 and the device wafer 100 form a base parallel to both wafers. The assembly comprising handle wafer 10 and the device wafer 100 is polished to achieve desired device thickness as shown in FIG. 1D. The device wafer 100 is then etched to form stand-offs 73 as shown in FIG. 1E. The stand-offs 73 are then covered by germanium 71 as shown in FIG. 1F. The device wafer 100 is then etched to form portions of MEMS device 110 (flexibly connected to the anchor) and 120 (rigidly connected to the anchor), anchoring points 130 for MEMS devices, and flexures 111 suspending the MEMS device 110 to the anchoring points 130 as shown in FIG. 1G. The MEMS device 110 and MEMS anchoring points are connected together through flexures substantially stiff in the direction perpendicular to the base. The condition on stiffness is important to prevent substantial movement of the MEMS device in the direction perpendicular to base during the eutetic bonding step. As shown in FIG. 1H, the handle and the device wafers are then eutetically bonded to the ASIC wafer 50 with exposed aluminum 72 at bonding points. Handle wafer 10 may be referred to as a top substrate and ASIC wafer 50 may be refereed to as bottom substrate.
FIG. 2 illustrates a typical anchor resulting from the Nasiri fabrication method. The anchor comprises an anchoring point 130 realized within the actuator layer which is basically the device wafer. The anchoring point 130 is rigidly connected to the top substrate 10 through the top substrate post, or short-post 20. The anchoring point is rigidly connected to the bottom substrate 50 through the eutetic bonding post comprising euteticly bonded germanium 71 and aluminum 72 and stand-off 73. Functionality of the anchoring pillar is two-pronged: it provides mechanical anchor for the MEMS device and, at the same time, electrical contact between the MEMS device 110 and bottom substrate 50 being an ASIC wafer. The problem with this two-sided anchoring is that external forces acting on the top substrate 10 may induce shear stress on the anchoring pillar and may substantially degrade the performance of the MEMS device.
Unlike the two-sided anchor shown in FIG. 2, a typical MEMS anchors fabricated in surface micromachining shown in FIG. 3 does not have such a problem. Surface machining anchor is rigidly connected only to one wafer 50. Lack of the post 20 prevents shear stress from acting upon the anchor 130 and MEMS device 110.
In particular, U.S. Pat. No. 7,478,557, entitled “Common centroid micromachine driver” discloses various types of MEMS anchors as well as anchoring suspensions. These types of MEMS anchors are related to improved package and over-temperature performance of the structures but they are addressing a different problem—a one-side anchor, as shown in FIG. 3.
U.S. Patent publication application 20070119252 (U.S. Pat. No. 7,430,909) discloses a MEMS accelerometer, and some of the disclosures are related to the improved package and over-temperature performance improvements. However, it is also related to the one-sided anchor of FIG. 3.
Accordingly, what is desired is anchor design that addresses the disadvantages of the Nasiri fabrication design and appears similar to the single sided anchor design while retaining the benefits of the Nasiri fabrication technique. The present invention addresses such a need.